Design of experiments to address manufacturing tolerances and process variation influencing cogging torque and back EMF in the mass production of the permanent magnet synchronous motors

A number of manufacturing challenges exist in creating a robust permanent magnet synchronous motor (PMSM) design to meet critical requirements for cogging torque and torque ripple in specific applications. In addition, reduction of back EMF harmonics that lead to torque ripple is also required during design. Due to manufacturing tolerances, maintaining capability with these requirements is very difficult especially for mass production of units. A design of experiments (DOE) that combines manufacturing tolerances with assembly process variations is proposed here to address the sensitivity of cogging torque and harmonics. The limits for the parts and the process variations are extracted from an actual production environment. The DOE is laid out according to one of the robust engineering matrices chosen from Taguchi´s method. The findings of the DOE are then utilized to set the tolerance limits for manufacturing the rotor magnets and to set the allowance for the rotor assembly process variation. The method helps limit the cogging torque and harmonics within predefined bands and make the motor design robust. This paper also investigates the capability of maintaining the tolerances of magnets for a surface mounted PMSM. The analysis is done using finite-element method and verified with experimental results.